Recently, the Internet protocol (IP) has been used on a number of communication bases regardless of a fixed communication and a mobile communication. In the field of mobile communication, a multifunction mobile telephone capable of implementing various application programs which communicate data by connection to the Internet has become widespread. Such a multifunction mobile telephone is referred to also as a smart-phone.
The above-mentioned multifunction mobile telephone may implement phone call application software using voice over IP (VoIP) and IP television telephone application software. For communications using such application software, a short packet of 64 bytes, 128 bytes, etc. is used to suppress a transmission delay. Since the amount of transmittable data by using a short packet is small, a large number of packets are used to transmit data. With the above-mentioned application software, a control signal is periodically communicated in the period in which a user data signal is not communicated. As a result, with an increasing use of application software by a multifunction mobile telephone, the traffic in a communication system increases.
In the communication system, an IP packet of a user data signal, a control signal, etc. may be transmitted after being encrypted for security. The encryption protocol may be, for example, security architecture for Internet protocol (IPsec). The encryption of data by the IPsec is performed in an IP layer, and the communication according to the application software, which is in a layer that is higher than a layer of the IPsec, is performed without recognition of the encryption by the IPsec. The process load of the device such as a security gateway etc. which performs the encrypted communication processing of an IP packet increases with the increase of the traffic in the communication system as described above.
The following technology is known as the technology of reducing the process load of the device which performs the encrypted communication processing of an IP packet. That is, in the IP network system having a security function, a transmitting device fragments packet data into a plurality of packets, and judges whether or not each of the plurality of fragmented packets is a packet targeted for encryption and transmission. Then, the transmitting device applies the IPsec communication only to a packet targeted for encryption and transmission in the plurality of fragmented packets, and transmits the other packets without encryption.
In the above-mentioned well known technology, the transmitting device only encrypts and transmits a part of packet data which is fragmented into a plurality of packets, and transmits the other fragmented packet data without encryption. That is, in the above-mentioned well known technology, all packet data is transmitted in a communication system, thereby not suppressing the increase of traffic in the communication system.